Many salt farms in India are located in the remote desert area known as the Little Rann of Kutch. These salt farms are typically manned by marginal workers and are mostly devoid of basic facilities such as electricity, transportation, clean water, etc. In these areas, the available ground water is salty (brine) which is a gift of nature for salt manufacturers. However, this invites the other difficulty with challenges for survival due to scarcity of drinking water. The salt producers sometimes have to travel long distances in search of potable water. Occasionally, drinking water is provided through tankers from sources far away from the salt farms. The supply, unfortunately, is not always regular and there are times when the land becomes so slushy that vehicles cannot ply. There is thus a need for an alternative and more dependable solution.
Techniques of desalination such as reverse osmosis and electrodialysis are good solutions when the water has salinity below that of seawater but become impractical solutions when the salinity of water is high as is the case with most sub-soil brines. Moreover, being in distant locations, salt farms often do not have electricity connection whereas power is required to drive the above units. On the other hand, operation through solar power, etc., is a costly proposition even if it were to be technically feasible.
The normal thermal desalination techniques require large inputs of fuel besides high capital cost and cumbersome size. These too are therefore unsuitable for such remote locations and unsophisticated users.
Solar stills have been used occasionally in salt farms to convert the saline water into drinking water. In such stills the average production rate is around 0.8 liters per square meter per sun hour. These could not become much popular in salt farms due to their low productivity.
The essential quality of heat is not the amount but rather its “value”. The strategy of how to recover this heat depends in part on the temperature of the waste heat gases and the economics involved.
Large quantity of hot flue gases is generated from boilers, kilns, ovens and furnaces. If some of this waste heat could be recovered, a considerable amount of primary fuel could be saved. The energy lost in waste gases cannot be fully recovered. However, much of the heat could be recovered and loss minimized by adopting suitable techniques.
Industrial units are typically designed incorporating heat recovery units to improve the overall thermal efficiency of the system. A common utilization of this principle is in systems which have an exhaust stream or waste stream which is discharged from the system to its surroundings. Thermal energy is often recovered from liquid or gaseous waste streams to fresh make-up air and water intakes in buildings, such as for heating, ventilation and air conditioning (HVAC) systems, or any process systems.
Typical waste heat sources and their temperature range released in atmosphere are given in Table 1. Flue gas at 450° C. is used in the Waste Heat Recovery Boiler (WHRB) to produce process steam. The large quantity of lower temperature waste heat from the engine cooling system (jacket cooling water, oil cooler and inlet air cooler) will be used to preheat to about 110° C. the feed water to the WHRB to increase its efficiency and produce more steam. The existing cooling tower will be replaced with heat exchangers and a de-aerator will be added for non-condense gases removal and further preheating of the WHRB feed water temperature to 130° C. which increases the system efficiency even more. The waste heat in the form of flue gases from thermal power plants and decentralized electricity generating sets are employed occasionally for processes requiring thermal energy. This directly reduces fuel costs and combustion emissions significantly, and further benefits the environment, albeit to a lesser degree, through reduced amount of effluents and reduced exhaust temperatures [Subramanyam, Waste heat recovery, Bureau of Energy Efficiency, February 2005, pp. 1-18].
TABLE 1Typical waste heat sources and temperatureSourceTemperature (° C.)Steel heating furnaces 925-1050Steam Boiler Exhausts230-480Gas Turbine Exhausts370-540Reciprocating engine exhausts325-600Heating treating furnaces425-650Internal Combustion Engines 66-120Hot Processed Liquids 32-232Welding Machines32-88Air compressors27-50pumps27-88
Reference may be made to Journal “Usability of low temperature waste heat for sea water desalination”, 1981, 39, 147-158, Risto Saari, a multi-effect-distillation unit is used for desalination of sea water using waste heat having 50° C. higher temperature than atmosphere. The energy that is cooled away from a process—appears at two very different temperature levels. This article states that even for temperatures less than 20° C. above the ambient temperature, waste heat can be technically and economically utilized.
Many other investigators have used waste heat to produce make-up water for different purposes like make-up, heating, cooling and drinking from sea water. Raha et al. (International Journal of Nuclear Desalination—2007, 2, 342) utilised waste heat to produce desalinated water by low-temperature evaporation (LTE) desalination technology. Low-pressure steam (0.13 bar) and even hot water (ΔT˜50° C.) has been used to produce high-purity water directly from seawater. LTE technology has found major applications in nuclear reactors to produce high-quality desalted water for make-up water requirements.
Reference may be made to US Patent Application No. 2007084778 A1 by S. T. Germain discloses a power generation system consisting of gas turbine for the production of electricity. A desalination system for production of potable water from sea water is attached to the exhaust of gas turbine. This desalination system comprises heat recovery steam generator and a condenser to condense the steam. Thus the system has dual functioning like power generation and to create a source of fresh water from sea water.
Reference may be made to Chinese patent No. CN201660459 by Huang discloses sea water desalination using the engine exhaust heat interchanger by overheating the sea water used for engine cooling. The water is circulated using a pump. The unit is claimed to be useful in ships.
Reference may be made to the web site http://www.brighthub.com/engineering/marine/articles/29189.aspx discloses a system wherein hot water from water cooled diesel engine is passed through the evaporator which is at low pressure and steam is generated. This steam is passed through steam separator and further passed to condenser to get fresh water which is extracted with the help of a pump.
Although the prior art above teaches us the utilization of waste heat for desalination, the units are typically fairly large units having sophisticated operation and requiring additional devices/electrical power for running different types of pumps. None of the prior art teaches the utilization of waste heat from small capacity air cooled diesel engines for desalination of highly saline—up to 3-5 times seawater salinity—sub-soil brine for the purpose of meeting drinking water needs of marginal salt workers in cost-effective manner in remote locations who have to otherwise struggle to procure drinking water.
Diesel engines are used to run pumps in salt works mainly to pump the sub-soil brine from high depths. Invariably all salt workers in places such as the Little Rann of Kutch possess such diesel engines to conduct their work of producing salt. The exhaust gas of a typical diesel engine is emitted at temperatures in excess of 150° C. and no use is made presently of this energy in such salt works. The present invention discloses the design of a device which enables this energy to be utilised gainfully for the production of potable water from highly saline brines in cost effective manner and at a rate in excess of that achievable through a solar still of similar size.